The stimulation of the human is the process by which the energy from a source in a kind of periodic alterations of the energy flow impacts on the human body, usually the skin or a body segment. Alterations and redistribution of mechanical, thermal, electrical or electromagnetic energy (called stimulus) are transformed by sensory receptors of the skin into feelings interpreted as tactile information (periodic heat, skin displacements/stretch, sense of pressure/force, pushes or pulses and vibrations, squeeze, tickling, tingling). Feelings associated with physical processes can take place or be generated in the field of contact, beneath or above the surface.
The need for the use of tactile information channel and simulation of the tactile feelings led to development of tactile actuators (thermal/infra-red, pneumatic, ultrasonic, electromagnetic, hydraulic, electrical and mechanical). However, since there are intermediate components between the source (actuator) and the specific receptors in the skin, a signal traveling from the source to the specific receptors may be lowered in terms of magnitude, and may be changed in terms of phase due to impedance of each intermediate component. Such distorted signal may be easily affected by external noise. These factors affect on transmission/propagation of tactile stimuli through different materials and substances having different structure and physical properties that can alter/dissipate energy of stimuli by making tactile signals weak and less informative as expected.
To account for noise and other disturbances, it was adopted that the energy alteration of applied tactile stimuli should exceed 24 dB above the sensitivity threshold for hairy skin (e.g., Mortimer B. J. P. et al. “Vibrotactile transduction and transducers” J. Acoust. Soc. Am., 2007, 121(5), 2970-2977).
Some efforts have already been undertaken to change the conditions for propagation of mechanical energy of tactile stimuli to skin receptors (e.g., De Rossi D., et al. “Electroactive polymer patches for wearable haptic interfaces” Conf. Proc. IEEE Eng. Med. Biol. Soc., 2011, 8369-8372, Carpi F. et al. “Electroactive Polymer-Based Devices for e-Textiles in Biomedicine” IEEE Trans. on Information Tech. in Biomedicine, 2005, 9(3), 295-318, Kim U. et al. “A transparent and stretchable graphene-based actuator for tactile display” Nanotechnology, 2013, 24, 145501, U.S. Pat. No. 7,375,454-B, U.S. Pat. No. 8,362,882-B) by placing actuators in a direct contact with human skin (smart fabrics/e-textiles and coverings), through compensation/suppression of disturbances, external noise and surround vibrations by making an exact (easy distinguishable) waveform of stimuli in a specific location due to detection of tactile stimuli propagation to a destination field of contact (e.g., U.S. Pat. No. 8,378,797-B), or by observing the result of skin deformation (variations in skin strain) in the field of contact and adapting the applied magnitude of tactile stimuli (e.g. U.S. Pat. No. 7,077,015-B). However, when the skin deformation occurs, that is, when e.g., fingers grip a rigid surface or fingertips act against a rigid surface or froze, protected with gloves, the skin receptors may be blocked even for higher level energy alterations which significantly exceed 24 dB above the skin sensitivity threshold, thereby making the proposed solutions inefficient.
Another way of improving the response of the human skin consists in altering sensitivity of skin receptors. Inventions, which relate to improving the sensory parameters of touch, in particular, to lower the threshold of skin receptors, have been disclosed in U.S. Pat. No. 5,782,873-A and U.S. Pat. No. 6,032,074-A. The method includes locating a receptive area where the function of receptors should be enhanced and applying a bias signal to this (skin) area before the informative (tactile) signals will be presented, perceived and identified. At that, the bias signals might have the same or different nature such as non-specific electrical or mechanical (gas/air flow) stimulation, than informative tactile signals. Such an approach can be efficient with optimal parameters of bias signals which have to be calibrated in advance. Nevertheless, parameters of the skin vary significantly and affected by many different factors of physical, physiological (humoral), and psychological nature. Therefore, it is difficult to predict whether a sensitivity change will happen or not within the predefined time interval and such a technique cannot easily be realized in practice. U.S. Pat. No. 8,040,223-B also discloses a method that includes the steps of temporarily altering the threshold of vibrational detection prior to the onset of tactile stimulus to achieve improved detection of the vibrotactile alert or communication signals without increasing the vibratory displacement amplitude. However, such an approach does not eliminate the problems of signal propagation to tactile receptors for sub-sensory vibrational stimuli that has to change sensitivity of the skin within the predefined time interval. Skin sensitivity depends on different factors of physical, physiological (humoral), and psychological nature. This approach is also constrained by specific parameters of vibration and conditions of tactile stimulation.
U.S. Pat. No. 8,253,703-B discloses a tactile interface that includes a plurality of individually controllable piezoelectric drivers positioned around a perimeter of a highly tensioned elastomeric material such as silicone rubber, polybutadiene, nitrile rubber, as well as other rubbers and elastomers. Driver circuitry can apply control information to each of the plurality of individually controllable drivers to produce a wave pattern in the tensioned elastomeric material. However, interaction through elastomeric material covering a stiff surface and having a density higher than human skin will squeeze the skin and increase the perceptual threshold by damping the response of skin receptors to tactile stimuli. Depending on a loss modulus, elastomeric materials may absorb the exerted energy to thereby alter the value and sense of the applied stimuli.
Another technical solutions are overlays and coverings, which allow to adjust a density of the surface of interaction. In particular, deformable overlays have been initially designed to detect the pressure and position of the fingertip on CRT displays, as disclosed in U.S. Pat. No. 4,542,375-A and U.S. Pat. No. 4,816,811-A, then to improve different strength and force envelops on the fingertip when pressing virtual keys of on-screen keyboards (e.g., US-2012-328349-A and Arai F. et al. “Transparent tactile feeling device for touch-screen interface” Proc. of the 2004 IEEE Int. Workshop on Robot and Human Interactive Communication, 2004, 527-532). The overlays and coverings can be filled in with a liquid or gel-like substance having a density similar to the density of hypodermis of the human skin, which is typically about of 1100 kg/m3 (e.g. Gennisson, J.-L. et al. “Assessment of Elastic Parameters of Human Skin Using Dynamic Elastography” IEEE Trans. on Ultrasonics, Ferroelectrics, and Freq. Control, 2004, 51(8), 980-989). However, these solutions have fixed/static parameters and do not allow changing them to control the result and efficiency of tactile stimulation.
In recent years, the advancements in robotics also enhanced the research and development of the soft artificial skins with multi-modal sensing capability (e.g., Park Y. et al. “Soft Artificial Skin with Multi-Modal Sensing Capability Using Embedded Liquid Conductors” IEEE Sensors, 2012, 12(8), 2711-2718, U.S. Pat. No. 8,033,189-B, U.S. Pat. No. 7,887,729-B, U.S. Pat. No. 7,740,953-B), and even having embedded elastomeric actuation points to simulate skin movements of facial expression (e.g., U.S. Pat. No. 8,568,642-B). However, a functionality of artificial skins is limited to sensing the contact event and actuation for imaging the specific patterns (e.g., facial traits) that can be visually recognized by human in the context of interaction scenario. That is, artificial robotic skins still are not intended to support processing and conditions of human touch and cannot be used as efficient tactile imaging system to mediate tactile-based interaction of the human in different environments (aggressive, dangerous or in artificial reality).